Normal cell membrane function is essential for the integrity of living organisms. Cell fusion is a measure of plasma membrane activity, and occurs spontaneously in vivo, e.g., during fertilization and the formation of mature muscle cells. We propose to isolate a series of lymphoid cell mutants which will be altered in their ability to undergo cell fusion. The mutants will be compared with their normal, congenic parent in physiological and biochemical tests which will describe the underlying phenotypic defect. They will also be analyzed genetically for dominance characteristics and complementation. This series of mutants should help define the genetic control of essential steps in plasma membrane function during cell fusion. In addition, because of the natural relationship between all plasma membrane fusion processes, these mutants should also help to describe basic functional steps in endocytosis and exocytosis. An understanding of the genetics of membrane activity is of basic importance in understanding the biology of all higher organisms.